The present invention relates to a process for producing higher molecular weight amino compounds, in particular difunctional and/or higher functional amino compounds which are free from lower molecular weight amino compounds or have a greatly reduced lower molecular weight amino compound content. The present invention also relates to a process for the production of isocyanate addition products made with these modified higher molecular amino compounds.
Compounds containing higher molecular weight amino groups of the type obtained, for example, by hydrolysis of the corresponding NCO prepolymers to convert an NCO group into an NH.sub.2 group are valuable starting materials for the production of poly(urea)urethanes as modifying epoxide cross-linking agents and for the known applications. These amino compounds are preferably obtained by alkaline hydrolysis of the NCO groups of prepolymers which are obtained by conversion of polyisocyanates (particularly of aromatic and/or also (cyclo)aliphatic diisocyanates such as toluylene diisocyanate, diphenylmethane diisocyanate and isophorone diisocyanate) with polyether polyols or polyester polyols containing two or tree hydroxyl groups (including polylactone ester diols or polycarbonate diols) in NCO/OH ratios &gt;1, preferably from 1.8:1 to 2.5:1. NCO prepolymers of this type generally also contain certain proportions of monomeric, lower molecular weight polyisocyanates (for example, toluylene-2,4-diisocyanate diisocyanate (TDI)), which change into monomeric, lower molecular weight amines (for example, 2,4-toluylenediamine (TDA)) during the production of the higher molecular weight amino compounds (i.e. amino polyethers or amino polyesters). The quantity of monomeric lower molecular weight amino compounds present depends upon the quantity of free monomeric polyisocyanate in the precursor compound (e.g. in the NCO prepolymer) but may also depend upon the type of reaction control during production and the working up operation used.
It is possible to avoid lower molecular weight monomeric amines (for example, TDA) in the so-called amino polyethers or amino polyesters or to keep them to a minimum by keeping the content of monomeric, lower molecular weight isocyanate (for example, TDI) low in the corresponding NCO prepolymers. This can be done, for example, by using a NCO to OH ratio of greater than 2:1 (for example, from 5:1 to 20:1) when forming the prepolymer and then distilling off (e.g. by means of a so-called thin film evaporator) the excess monomeric (di)isocyanate. Monomeric concentrations of less than 0.5 wt % monomeric diisocyanate may thus be obtained. However, these methods have the disadvantage of the high cost for thin film distillation of suitable NCO prepolymers.
In another process, NCO/OH ratios of less than 2:1 (for example, from about 1.75 to 1.95) which means less starting diisocyanate is present than would be used for an "ideal" NCO prepolymer (NCO/OH ratio 2:1) are used during prepolymerization to obtain a product which is low in monomers (allowing for a certain pre-extension of the NCO prepolymer). This process has the disadvantage that the NCO prepolymer as well as the amine conversion product obtained from it have high viscosities which make them difficult to process.
However, it is worthwhile and desirable to remove monomeric amine, in particular aromatic diamines from the so-called higher molecular weight amino polyethers or amino polyesters. More specifically, removal of monomeric amines brings the higher molecular weight amines within the limits for the concentration of lower molecular weight (in particular aromatic) amines established by the government. Removal of monomeric amine also eliminates the negative influence of free aromatic, lower molecular weight amines on the stability to light of the amino polyethers or polyesters, or compositions produced therefrom. Such removal also makes it possible to avoid variations in properties in polyurethane ureas made with such polyamines attributable to monomer content. Further, removal of monomeric amine increases the stability to light and to discoloration in polyurethane plastics produced from the higher molecular weight amine.